Computer systems are usually defined as having three main blocks: a central processing unit (CPU), memory, and input/output peripherals. Over the past half-century or so developments in integrated circuit technology have made computers inexpensive and therefore common in everyday life, for all types of uses including desktop personal computers, cellular telephones, automobile engine controllers, and the like.
Integrated circuit technology has greatly reduced the cost of all components of the computer system. For example CPUs are now implemented with single-chip microprocessors. Modern microprocessors can achieve computing performance that was only available to supercomputers just a generation ago.
The integrated circuit revolution has also reduced the cost of computer memory. FIG. 1 illustrates a computer system 100 known in the prior art. Computer system 100 includes a microprocessor 110, a memory 120, and an input/output system 130 all connected together by means of a system bus 140. As shown in FIG. 1 memory 120 includes several different types of memory devices, including a read-only memory (ROM) 122, a flash electrically-erasable programmable ROM (FLASH) 124, a static random access memory (SRAM) 126, and a dynamic random access memory (DRAM) 128. Computer system 100 requires some or all of these different types of memory devices because integrated circuit technology has so far failed to produce a single type of memory device that meets all the needs of computer system 100.
ROM 122 is nonvolatile, permanent storage that is inexpensive but cannot be re-written. ROM 122 is implemented using mask-programmable single-transistor memory cells and requires custom masks but is inexpensive to manufacture. ROM 122 stores a program executable by microprocessor 110 that is not expected to change over the lifetime of computer system 100. A typical use of ROM 122 is to store the basic input/output system (BIOS) of a desktop computer.
FLASH 124 is nonvolatile memory that may use floating-gate transistors to store electrical charges that indicate the state of the memory cells. Relatively large portions or sectors of FLASH 124 can be erased “in a flash”. However FLASH 124 is somewhat more expensive than ROM 122 and a program operation to write new data takes much longer than a read operation. Furthermore FLASH 124 has a reliability problem known as endurance, in that it loses its ability to be re-programmed over time after a large number of erase and program cycles have been performed. Because of these characteristics FLASH 124 is used to store parameters and user settings that stay relatively constant. For example, if computer system 100 were used in a cellular telephone, FLASH 124 could be used to store phone numbers in an electronic “phone book”.
SRAM 126 is randomly accessible at high speed for both reads and writes, but it volatile and relatively expensive. SRAM 126 includes memory cells implemented as addressable static latches. Thus it is usually reserved for use as a high-speed scratchpad memory or cache for microprocessor 110.
DRAM 128 is randomly accessible and inexpensive, but is slower than SRAM 126 and is volatile. DRAM 128 is commonly implemented using storage cells that each include a capacitor and a single transistor, and thus can achieve high density and low price per bit. However because the charges stored in the memory cells are dynamic, the memory cells must be periodically refreshed. DRAMs are commonly used for storage of large programs that are loaded from input/output devices such as compact discs (CDs) and hard disk drives.
It should be apparent that any actual computer system may not use all types of memory devices shown in FIG. 1.
In recent years semiconductor manufacturers have developed a new type of memory known as magneto-resistive random access memory (MRAM) that may one day unify the available types of memory devices. MRAM is based on small storage cells that store states using magnetic fields. MRAM is non-volatile, randomly accessible, and capable of high-density integration. However as of yet manufacturing techniques have not been developed to produce MRAM at costs comparable to FLASH or DRAM at comparable densities.
Therefore what is needed is a new memory system capable of taking advantage of the properties of MRAM while it remains more expensive than these other types of memory.